Molecular sieves of the crystalline aluminosilicate zeolite type are well known in the art and now comprise over 150 species of both naturally occuring and synthetic compositions. In general, the crystalline zeolites are formed from corner-sharing AlO.sub.2 and SiO.sub.2 tetrahedra and are characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent crystal structure.
Other crystalline microporous compositions are known which are not zeolitic but which exhibit the ion-exchange and/or adsorption characteristics of the zeolites. These include: 1) a pure silica polymorph, silicalite, having a neutral framework containing neither cations nor cation sites as disclosed in the U.S. Pat. No. 4,061,724; 2) crystalline aluminophosphate compositions disclosed in U.S. Pat. No. 4,310,440; 3) silicon substituted aluminophosphates as disclosed in U.S. Pat. No. 4,440,871 and 4) titanium substituted aluminophosphates as disclosed in U.S. Pat. No. 4,500,651.
Applicant has discovered a new class of molecular sieve materials which are a three-dimensional microporous crystalline zinc phosphate composition. These materials have a high framework charge and a high charge gradient which arises from the alternating Zn.sup.+2 and P.sup.+5 tetrahedral sites in the framework. They are capable of ion exchange, adsorption of gases and are useful as catalysts.
There are a number of reports dealing with the synthesis and characterization of zinc phosphate materials. For example the crystal structure of Zn.sub.2 HK(PO.sub.4).sub.2.2H.sub.2 O has been reported by M. T. Averbuch-Pouchot and A. Durif in J. Applied Crystallography, No. 7, p. 403 (1974). Further, the properties of KZn.sub.2 H(PO.sub.4).sub.2.2.5H.sub.2 O has been reported by T. Barbou des Courieres and M. H. Simonot-Grange in Materials Research Bulletin, Volume 12, pp. 355-360 (1977). In this article the authors report that the zinc phosphate material contains water molecules which have partially zeolitic properties. However, heating of this material at temperatures of about 450.degree. C. results in a collapse of the structure. More importantly, the structure of this material was determined by I. Tordjman et al. in Acta Cryst. B31, 1143 (1975) and showed that two of the zinc atoms in the unit cell were not tetrahedrally coordinated (See Table 4, pp. 1147-48). Therefore, this material does not have a framework made up of tetrahedrally coordinated zinc and phosphorus atoms. Other references include crystal structure of (Na,K) (ZnPO.sub.4) by O. V. Yakubovich and O. K. Mel'nikov in Sov. Phys. Crystallogr., 34(1) (1989) and A. W. Frazier et al. in J. Agr. Food Chemistry, Volume 14, page 522-529 (1966). However, both of the materials reported in these references are dense phase materials with the former reference stating that their material has a .beta.-tridymite structure.
At this point it is helpful to discuss the difference between "microporous" materials and "dense phase" materials. Microporous materials are materials which have an intracrystalline pore system. The pores are large enough to admit various gaseous or liquid molecules. One example of microporous materials are zeolites. Zeolites contain cations which are found in the intracrystalline pore system of the zeolite and are hydrated. Therefore, a traditional criterion for microporosity is the presence of intracrystalline waters of hydration which are associated with cations found in the pores, i.e., structure directing cations.
Microporous materials are metastable with respect to solid state or dense phases. What this means is that all microporous materials, as a consequence of their metastable nature, will display either a structure collapse or a structure collapse followed by a recrystallization to a dense phase before the melting temperature is reached.
Dense phase materials on the other hand do not have an intracrystalline pore system which is capable of admitting gaseous or liquid molecules. Although in certain crystallographic views, these dense materials may appear to have intracrystalline pore systems, these pores are only large enough to accommodate a small cation without any waters of hydration. That is, the cation is actually coordinated to the framework oxygen atoms. Further, dense phase materials will not undergo a complete structure collapse before melting occurs.
Applicant has synthesized a microporous zinc phosphate molecular sieve which has a three-dimensional microporous framework structure of ZnO.sub.2 and PO.sub.2 tetrahedral units. That is, all the zinc and phosphorus atoms are tetrahedrally coordinated. Although the des Gourieres reference states that their material has water which may be "zeolitic", the structure of their material shows that not all of the zinc atoms are tetrahedrally coordinated. Therefore, applicant is the first person to have synthesized a zinc phosphate molecular sieve in which all the zinc and phosphorus atoms are tetrahedrally coordinated.